(a) Field of the Invention
The present invention relates to a piston for direct injection internal combustion engines enables the engine to realize an lean burn.
(b) Description of the Related Art
Generally, internal combustion engines are operated by supplying an air/fuel mixture into a cylinder, and compressing and igniting the mixture. A procedure for generating power in internal combustion engines to drive a vehicle comprises the steps of supplying an air through air supply system, injecting fuel such that it can mix with the air during an intake stroke, spraying the air-fuel mixture into a vaporization portion, igniting the mixture using a spark plug, and exhausting burned gas through an exhaust system.
Recently, much research and development is being pursued for improving fuel consumption ratio and reducing emission utilizing the direct injection internal combustion engines.
As one type of such an engine having a direct fuel injection system, an engine having two intake ports, one of which includes a swirl control valve for regulating swirl in a combustion chamber, has been introduced for enhancing the efficiency of flame propagation.
An example of this type of engine is disclosed in U.S. Pat. No. 5,553,588. FIGS. 1 and 2 are respective front cross-sectional and top views of U.S. Pat. No. 5,553,588.
In FIG. 1, reference numeral 10 denotes a combustion chamber, 30 a piston, and 50 a vaporization portion formed in an upper surface of the piston 30. Reference numeral 20 in drawing denotes a fuel injection valve that directly injects fuel into the combustion chamber 10, and 40 is a spark plug. In addition, 70a and 70b, and 90a and 90b (only 70a and 90a are shown in FIG. 1.) indicate intake valves and exhaust valves, respectively. In such a conventional engine, since each cylinder has two intake valves and two exhaust valves, this configuration is known as a four-valve chamber.
As shown in FIG. 2, intake passages 110a and 110b are provided for supplying air into the combustion chamber 10. The intake passages 110a and 110b are connected respectively to intake ports 80a and 80b. The intake port 80a has a helical configuration that guide the flow of intake air in a spiral direction, thus serving as what is known as a swirl port. In contrast to the intake port 80a, the intake port 80b is a straight port through which intake air flows linearly into the combustion chamber 10. The intake passage 110b that is connected to the intake port 80b, i.e., the straight port, is provided with a swirl control valve 200.
With reference to FIG. 3, the vaporization portion 50 formed in the upper surface of the piston 30 has a first wall 50a, a second wall 50b and a third wall 50c. The first and third walls 50a and 50c are arcuate and disposed so as to be mutually opposing along the direction of the swirl flow. The first and third walls 50a and 50c are connected, on a side opposing the fuel injection valve 20, by the second region 50b.
In FIG. 4, showing another example of the piston 30 disclosed in the U.S. Pat. No. 5,553,588, a pocket 50d is formed in a portion of the vaporization portion 50 corresponding to approximately where the second wall 50b and the third wall 50c meet. The pocket 50d enlarges a capacity of the third wall 50c.
In FIG. 5 showing yet another example of the piston 30, a part 50e of the second wall 50b, which is positioned on downstream side of the swirl flow with respect to the spark plug 40, is formed inwardly toward the spark plug 40 so that a constant distance is maintained between the spark plug 40 and this area of the vaporization portion 50.
FIGS. 3, 4, and 5 illustrate primary stages at which fuel is injected into the vaporization portion 50.
The process of combustion of the air-fuel mixture according to the above described engine having a direct injection combustion chamber will be described hereinafter with reference to the drawings.
On the intake stroke the piston 30 moves toward the bottom dead center in the combustion chamber 10 by a crankshaft and connecting rod (not shown). During the intake stroke, the intake valve 70a is held open by the camshaft (not shown). Since the piston 30 moves down in the combustion chamber 10, a low-pressure area is created such that the air is forced past the intake valve 70a into the combustion chamber 10.
As the piston 30 moves toward the top dead center by the crankshaft from a bottom dead center, the intake valve 70a closes. The air is trapped in the combustion chamber 10 and therefore compressed by the piston moving upward. When the piston has reached near top dead center, a predetermined amount of fuel is injected into the combustion chamber 10 from the injector 20. The fuel is diffused and vaporized in the depressed vaporization portion 50 that the air-fuel mixture is concentrated around the spark plug 40 by the walls of the vaporization portion 50.
Now, the behavior of the fuel injected into the combustion chamber 10 of the direct fuel injection engine of the prior art will be explained in more detail.
The fuel injected into the vaporization portion 50 strikes on a bottom surface of the vaporization portion 50 to form a cloud of fuel mist. This fuel mist is carried by the swirl and flows along the arc-shaped first wall 50a. During the time the fuel mist flows along the first wall 50a, the fuel particles are diffused and vaporized to form the ignitable air-fuel mixture, which reaches the second wall 50b. Because the second wall 50b is an approximate straight line, the ignitable air-fuel mixture passes quickly along the second wall 50b to pass by the spark plug 40, after which the air-fuel mixture flows into the third wall 50c. However, since the capacity of the third wall 50c side of the vaporization portion 50 is larger than the capacity of the first wall 50a side of the vaporization portion 50, even when there is a large amount of fuel injected, the injected fuel is prevented from accumulating in the area near the spark plug 40. Therefore, an air-fuel mixture of a sufficient concentration is not formed in the area near the spark plug 40.
In the above state, the air-fuel mixture is ignited by a spark from the spark plug 40 and expansion of the burning mixture causes a rapid rise in pressure. This increased pressure forces the piston down on the power stroke, causing the crankshaft to rotate. At the end of the power stroke the camshaft (not shown) opens the exhaust valve 90a, and the exhaust stroke begins. Remaining pressure in the combustion chamber 10 and upward movement of the piston 30 force the end gases out of the combustion chamber 10. However, in the prior engine having the direct fuel injection system, since the injector 20 is oriented to the direction of the bore center of the piston 30 to spray the fuel to the first wall 50a of the vaporization portion 50, the fuel injected at the end of the compression stroke can overflow out of the vaporization portion 50 at the moment the fuel contacts to the surface of the vaporization portion 50 by spraying pressure. This in turn, causes irregular ignition such that it is impossible to obtain uniform expansion power.
Furthermore, because an adhesive effect, in which some of the fuel adheres to the wall of the vaporization portion 50, occurs while the fuel passes over the second wall 50b, problems result such as deterioration in stratification of the fuel and the realization of only partial vaporization. Accordingly, the piston is heated excessively, resulting in a lower air-fuel ratio than is ideal.